I. Field of the Invention
The present invention relates to a low-voltage (2 to 5 V), large-current (20 to 200 A) type forward converter primarily operated at a high frequency of 500 to 1,000 kHz and, more particularly, to a compact full-wave output type forward converter with high reliability and high efficiency.
II. Description of the Prior Art
FIG. 7 shows a typical conventional forward converter. A drain-source voltage Vq of a MOSFET 1 as a switching element is clamped by a reset winding 2 and a diode 3. Since this circuit outputs a half-wave signal, the ON time ratio of the FET 1 is limited to normally 45% or less, and a filtering effect at the output side is degraded. In addition, a chemical capacitor having a large capacitance and a large size must be used as a capacitor 5 in a filter 4. For this reason, the decreases in capacitance and size of the capacitor, which can be achieved by a high-frequency arrangement, are limited to a certain degree.
FIG. 8 shows another conventional forward converter in which energy obtained by clamping a drain-source voltage Vq is consumed by an impedance of a circuit consisting of a resistor 14 (Rc) and a capacitor 13 (Cc). The ON time ratio of an FET 1 can be increased to 50% or more. However, the peak value of the drain-source voltage Vq is increased in proportion to the output power. Therefore, this forward converter can be used only when a low output is required.
FIG. 9 is still another conventional forward converter introduced some time between 10 and 20 years ago. Although this converter aims at reducing the size of a filter 4 on the output side, an object of the present invention cannot be achieved. This conventional converter has never been popular in practical applications.
The problems posed by the conventional converters shown in FIGS. 8 and 9 will be analyzed.
An equivalent circuit of FIG. 8 can be illustrated as in FIG. 10. Reference symbol L.sub.1 denotes a leakage self conductance of a primary winding 6; and L.sub.2, a leakage self inductance of a secondary winding 7 which is calculated as a value on the primary side.
FIG. 11 is a graph showing a drain-source voltage Vq, a drain current Iq, and a transformer current It in the converter shown in FIG. 8. The graph in FIG. 11 will be described together with the circuit shown in FIG. 10.
When an FET 1 is turned off at time T.sub.1, theoretically, the drain current Iq is immediately decreased. The transformer current It is not cutoff by energy of {(L.sub.1 +L.sub.2).sqroot.Iq.sup.2 }/2 accumulated in the windings L.sub.1 and L.sub.2 and continuously flows through a diode 9 and a capacitor 11 in a snubber circuit 8 until time T.sub.3. In this case, equation (1) below can be established: EQU {(L.sub.1 +L.sub.2).multidot.Iq.sup.2 }/2=(Cs.multidot.Vqp.sup.2)/2 . . . (1)
for Vqp=Vcsp, and the voltages Vcs and Vq rise from zero at time T.sub.1. A resistor 10 has a resistance enough to discharge the voltage Vcs of the capacitor 11 to zero for a duration between time T.sub.4 and time T.sub.1. Excitation energy accumulated by L.phi. is discharged by a clamp circuit consisting of a diode 12 and a capacitor 13 for a duration between time T.sub.2 and time T.sub.3. In a practical circuit, at time T.sub.2 when Vq is maximized to Vqp, a charge current is supplied to the capacitor 13, and the transformer current It between time T.sub.2 and T.sub.1 cannot be observed. In this case, a resistor 14 serves as a discharge resistor for maintaining a voltage Vcc of the capacitor 13 at a safe value and thus has a high resistance.
When equation (1) is taken into consideration, condition Vqp.varies.Iq is established. In other words, the voltage Vqp is increased in proportion to Iq, i.e., a load current. This indicates that the conventional converter can be used in only a low-power arrangement. An increase in capacitance of the capacitor 11 allows a decrease in Vqp. However, this indicates a loss in the resistor 10 during discharging of the capacitor 11 for a duration between time T.sub.4 and time T.sub.1. Therefore, a sufficiently large capacitance cannot be usually used.
Equation (1) will be further examined. Condition L2 &gt;L1 is generally established in a low-voltage output circuit. If energy accumulated by L.sub.2 is set so as not to influence an increase in Vq, the value of the right-hand side of equation (1) is apparently decreased together with both the values of Cs and Vqp.
The conventional converter in FIG. 9 can more or less suppress the influence of L.sub.2. However, as compared with the conventional converter in FIG. 8, N.sub.2 (FIG. 9) =N.sub.2 /2 (FIG. 8) is established with identical output voltages. Therefore, L.sub.1 is increased by an increase in N.sub.1 /N.sub.2 winding ratio, and the voltage Vqp is increased.
In summary, in the conventional arrangements, the voltage Vqp is increased in proportion to the output voltage to cause an increase in a voltage stress acting on the switching element, thereby degrading circuit reliability. If the capacitance Cs is increased in order to prevent the above problem, the internal loss is increased and energy conversion efficiency is degraded. Therefore, the conventional converters are used as only low-power converters.